This invention relates to hardness testers and particularly to an instrument for measuring coating hardness.
One such device is the Hoffman Scratch-Hardness Tester sold by Gardner Laboratory Incorporated (Catalog No. SG-1610-M). The original tester is described in U.S Pat. No. 2,279,264 and the current model is a slight modification of this device. It consists basically of a four-wheeled carriage; a scale arm graduated from 0-20 that is attached permanently to the carriage in a counter-poised condition about the pivot axis; and a scratching tool with a sharp circular mounted mouned at 45.degree. to the test surface. In operation small and/or large riders are attached to the scale arm at the numbered positions. The carriage is held down firmly by hand and moved in the opposite direction to cause a trailing scratch. The larger rider generates a load of 100 grams per division, while the small rider generates a load of 25 grams per division. The tester is placed on a reasonably flat, level, test surface that measures at least 2 in. .times. 4 in.
The thumb and first two fingers of the right hand are placed on the carriage platform, directly over the pair of wheels nearest the scratching tool, using sufficient downward pressure to lift the counterpoised beam with the selected rider at the 0 position. The tester is pulled to the right in a straight line across the test surface, making sure that the scratching tool follows the rolling carriage.
The scratch resistance is measured by increasing the load by moving the rider to the left and observing the load necessary to make just a preceptible mark or scratch on the coating. As an alternate procedure, the width and character of the scratch made on a series of test surfaces with the same load are compared. A measuring microscope facilitates the measurement.
However, in use, polymer scratched from the coating often accumulates on the scratching tool causing erratic results and poor scratch definition.